Antimicrobial filtration membranes

ABSTRACT

A method for in situ production of antimicrobial filtration membranes that uses self-assembly of surfactants such as block copolymers as a template. The mesophase structure (for example hexagonal or lamellar) can be determined, and membrane pore size can be controlled in the nanometer range, by changing the block copolymer and the amounts of the components such as the block copolymer, aqueous solution, monomer, crosslinker, and initiator. The monomer phase cures in the template and there is no need for organic solvents and coagulation bath or other post-modification. As-synthesized membranes were found to have pore sizes with a narrow size distribution in the range of 3-4 nm with a molecular weight cutoff of 1500 g/mol and displayed both excellent fouling resistance and high permeance of water, vastly outperforming a conventional NIPS UF membrane. The monomer can comprise a quaternary ammonium group so that the membrane is antibacterial. The block copolymer can comprise hydrophilic blocks which form the surfaces of the membrane pores, rendering them hydrophilic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/261,496, filed on Jan. 29, 2019, entitled“Antimicrobial Filtration Membranes”, issuing on Sep. 6, 2022 as U.S.Pat. No. 11,433,359, which application claims priority to and thebenefit of the filing of U.S. Provisional patent application No.62/623,321, filed on Jan. 29, 2018, entitled “Antimicrobial FiltrationMembranes”, and the specification and claims thereof and appendicesthereto are incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of CooperativeAgreement No. R10AC80283 awarded by the United States Bureau ofReclamation.

BACKGROUND OF THE INVENTION Field of the Invention (Technical Field)

The present invention is related to a process for the production ofantimicrobial filtration membranes using a templating approach and insitu formation of a mesoporous polymer, and membranes produced thereby.

BACKGROUND ART

Note that the following discussion may refer to a number of publicationsand references. Discussion of such publications herein is given for morecomplete background of the scientific principles and is not to beconstrued as an admission that such publications are prior art forpatentability determination purposes.

Ultrafiltration (UF) and microfiltration (MF) are among the mostcommonly employed separation techniques with applications in a varietyof industries ranging from food processing to chemical manufacturing andprotein purification. In the treatment of water and wastewater, UFand/or MF play a key role in the removal of suspended particles,viruses, and bacteria. Conventionally, these membranes are produced bynon-solvent induced phase separation (NIPS), a method that isenvironmentally questionable due to its use of large quantities oforganic solvent, approximately 70% (by volume). Additionally, NIPSmembranes are anisotropic with low surface porosity, leading toincreased fouling on the surface of the membrane. Bio-fouling is acommon problem found in membrane filtration as proteins, bacteria, andviruses accumulate on the surface of membranes. As a result, there hasbeen a great deal of effort to develop improved UF membranes viamodification of the NIPS method and resultant membranes as well as thedevelopment of alternative UF membrane production routes.

The most common approach taken in the development of improved UFmembranes has been surface modification. Grafting hydrophilic groups,such as poly(ethylene glycol), to the surface of membranes has beenproven to reduce fouling. However, surface modification is costly.Another approach, a combination of polymer self-assembly and NIPS,referred to as SNIPS, affords resultant membranes with high flux andanti-fouling properties. In this method, immersion of a self-assembledblock copolymer film in a non-solvent bath leads to the formation ofhierarchically porous membranes. However, the SNIPS method stillrequires large quantities of organic solvent and, as such, is noteco-friendly. Templating is an alternative route to porous membranes inwhich a structured or porous material is used as a template to impartstructure to another material and subsequently removed. Consequently,the pore size and pore size distribution can be readily controlled bychanging the template. Surface-active agents (surfactants) canself-assemble in water/oil mixtures to form anisotropic, mesomorphicphases (mesophases) exhibiting high extension in one or two dimensions,making them ideal templates for membranes. These mesophase templates arethermodynamically stable systems with length scales on the order of 2-50nm, and can be prepared through polymerization of mesoporous polymers. Acritical problem observed in this templating approach ispolymerization-induced phase separation/inversion in which the templatestructure is not retained after polymerization. The use of amphiphilicblock copolymers as surfactants has been shown to improve the retentionof the template structure due to the slow kinetics of block copolymers.However, most of the reported lyotropic liquid crystal (LLC) templatingtechniques found to retain the LLC structure after polymerizationrequire polymerizable surfactants prepared via complex chemistries.

In addition, bio-fouling is a common problem found in membranefiltration as proteins, bacteria, and viruses accumulate on the surfaceof membranes. Currently, chlorination is utilized in municipal watersystems to remove tiny micro-organisms and bacteria. However, harmfuldisinfection byproducts produced during the chlorination process haveraised concerns and motivated exploration of other disinfection agents.Therefore, if combined with disinfection, ultrafiltration andmicrofiltration can transform not only municipal water treatment, butalso the treatment of wastewater containing harmful micro-organisms andbacteria due to their high flux rate and efficiency. In recent years,antibacterial membranes have attracted industrial and academic interestfor the removal of bacteria and micro-organisms from water.Conventionally, antibacterial membranes are prepared through surfacemodification, but most surface modification routes are limited tospecific types of membranes. Furthermore, these routes require the useof complex and often expensive chemical reactions to graft antibacterialgroups onto the surface, making the final product costly.

SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)

The present invention is a method of manufacturing a membrane, themethod comprising mixing a surfactant, an aqueous solution, and amonomer solution to form a mesophase; coating the mesophase on a poroussupport; and polymerizing the mesophase to form a porous membrane. Theporous support is preferably nonwoven. The method is preferablyperformed without the use of an organic solvent. The method preferablyfurther comprises removing the aqueous solution after the polymerizingstep. The monomer solution preferably comprises a monomer, a crosslinkerand an initiator. The monomer is preferably functionalized with anantibacterial group, which preferably comprises a quaternary ammoniumgroup. The initiator is preferably a thermal initiator or aphoto-initiator. The surfactant preferably comprises a block copolymer.The block copolymer preferably is a block copolymer of poly(ethyleneoxide) and poly(propylene oxide). The block copolymer preferablycomprises hydrophilic blocks and hydrophobic blocks. The hydrophilicblocks preferably form the surfaces of pores in the membrane. Thesurfactant preferably does not comprise a small molecule surfactant. Theaqueous solution is optionally deionized water. The mixing step isoptionally performed via centrifugation. The polymerizing steppreferably comprises exposing the mesophase to ultraviolet radiationand/or heating the mesophase to a temperature below 100° C. for lessthan 5 hours. The membrane is preferably antibacterial. The methodpreferably comprises choosing relative amounts of the surfactant, theaqueous solution, and the monomer solution in order to produce a desiredstructure of the mesophase, such as hexagonal or lamellar. The methodoptionally further comprises hot pressing the mesophase and the supportto infuse the mesophase into the support prior to the polymerizationstep. The pore size of the membrane is preferably less thanapproximately 5 nm. The average grain size of the membrane is preferablyapproximately 100 nm. The membrane preferably comprises monodispersepores and/or is isoporous. The surfaces of the pores are preferablyhydrophilic.

Objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate the practice of embodiments of thepresent invention and, together with the description, serve to explainthe principles of the invention. The drawings are only for the purposeof illustrating certain embodiments of the invention and are not to beconstrued as limiting the invention. In the figures:

FIG. 1 is a schematic representation of an antimicrobial membraneproduction line of the present invention.

FIG. 2 shows the chemical structure of [2-(acryloyloxy) ethyl]trimethylammonium chloride.

FIG. 3 shows the dilution steps used for making the PetriFilm samples ofExample 1.

FIGS. 4A-4D show results from the growth of E. coli colonies onPetriFilms as described in Example 1 for: an as-synthesized membranewith a 1:1 dilution of wastewater (FIG. 4A), an as-synthesized membranewith a 1:10 dilution of wastewater (FIG. 4B), a control sample with a1:1 dilution of wastewater (FIG. 4C), and a control sample with 1:10dilution of wastewater (FIG. 4D).

FIG. 5 shows schematic representations of a templating method of Example2 for making mesoporous polymers using lamellar (top row) and hexagonal(bottom row) mesophases. The oil phase comprised monomer, crosslinker,and initiators. Lattice parameters, apolar and polar domain sizes, andpore sizes are shown.

FIGS. 6A and 6B show 1-D SAXS profiles of the mesophase system ofExample 2 comprising monomer (Pluronic L64/water/(butylacrylate+EGDMA+HCPK+AIBN)) after polymerization, with and without asupport, respectively.

FIG. 7 shows cross-polarized light micrographs obtained for mesophasesbefore and after polymerization with compositions I, II, III, and IV, aslisted in Table 2. Scale bar: 50 μm.

FIGS. 8A and 8C show 1D and 2D scattering profiles for composition I andIII mesophases, respectively, before polymerization. FIGS. 8B and 8Dshow 1D and 2D scattering profiles for composition I and III mesophases,respectively, after polymerization. The compositions are listed in Table2.

FIGS. 9A-9D show 1D and 2D SAXS profiles for mesophase systemscontaining a monomer comprising Pluronic L64/water/(butylacrylate+EGDMA+HCPK+AIBN). FIG. 9A shows the 50/35/15 composition beforepolymerization; FIG. 9B shows the 50/35/15 composition afterpolymerization; FIG. 9C shows the 55/15/30 composition beforepolymerization; and FIG. 9D shows the 55/15/30 composition afterpolymerization.

FIG. 10 shows a log-log scale 1D SAXS profile for a mesophase systemcontaining a monomer (Pluronic L64/water/(butylacrylate+EGDMA+HCPK+AIBN)) with a 60/15/25 composition beforepolymerization. Pre- and post-shoulders in primary peak at 0.91q* and1.15q* are indicative of HM/HPL structures.

FIG. 11 shows 1D and 2D SAXS patterns as a function of temperature forsample I with composition of (Pluronic L64/water/(butylacrylate+EGDMA+HCPK+AIBN) (60/30/10).

FIG. 12 is a schematic representation of grain boundaries that can leadto open or dead-end pores.

FIG. 13A shows a UV-Vis spectra of feed solution, permeate of membrane I(lamellar), and permeate of membrane III (hexagonal). FIG. 13B shows aUV-Vis calibration curve of BSA.

FIGS. 14A-14B show filtration of a 1 mg/mL BSA solution (FIG. 14A), anda 1 mg/mL PEG300 solution (FIG. 14B) through lamellar (I), hexagonal(III), and commercial membranes. Error bars indicate the standarddeviation of three measurements.

FIG. 15A shows the MWCO curve using the TOC method for an as-synthesizedlamellar membrane (sample I); FIG. 15B shows the MWCO curve using theTOC method for an as-synthesized hexagonal membrane (sample III).

FIG. 16 is a cross-sectional SEM image for membrane I showing an averagethickness of 10±2 μm.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are methods for preferably one stepproduction of UF and MF antimicrobial filtration membranes via in situpolymerization of one or more functionalized monomers that preferablyuse self-assembly of block copolymers as a template. Membrane pore sizescan be controlled in the range of 2-50 nm by changing the blockcopolymer and the ratio of components. The monomer phase cures in thetemplate and there is no need for organic solvents or a coagulationbath. The membranes can be scaled-up for industrial purposes as well asadapted in small portable units for production in rural areas and smallcommunities. The templating approach provides improved permeability overother methods, and enables flexibility in producing membranes withdifferent chemistries and mechanical properties. The characteristics offinal membranes may be varied depending on the composition andcharacteristics of the components. Antimicrobial membranes produced fromthe method of the present invention are useful for municipal water andwastewater treatment facilities, food industries, biomedicalapplications, and pharmaceutical industries. The membranes of thepresent invention preferably do not comprise silver nanoparticles.

The template preferably comprises three main components: a surfactantphase, an aqueous phase, and an organic or oil phase. In general, themonomer phase (which may be the organic/oil or aqueous phase) preferablycomprises mixtures of monomers, crosslinkers, and thermal and/or photoinitiators. Preferably no organic solvent is necessary to producemembranes of the present invention. The surfactant phase preferablycomprises approximately from 20-80 wt %, more preferably approximatelyfrom 30-70 wt %, and most preferably approximately from 50-60 wt % ofthe mixture. The aqueous phase (such as, but not limited to, water,distilled water, deionized water, or an aqueous solution) preferablycomprises approximately 0-40 wt %, more preferably approximately from10-35 wt %, and most preferably approximately from 15-30 wt % of themixture. The oil phase preferably comprises a monomer that is liquid atthe mixing temperature and pressure, as well as one or more crosslinkersand one or more initiators, preferably comprises the remainder of themixture, that is preferably approximately from 1-80 wt %, morepreferably approximately from 5-60 wt %, and most preferablyapproximately from 10-35 wt % of the mixture. The oil or organic phasepreferably comprises a crosslinker approximately from 10-100 wt %, morepreferably approximately from 20-60 wt %, and most preferablyapproximately from 30-50 wt % of the oil phase. An initiator preferablycomprises approximately from 0.1-10 wt %, more preferably approximatelyfrom 1-5 wt %, and most preferably approximately from 3-5 wt %. Theremainder of the oil phase preferably comprises the monomer; that ispreferably approximately from 1-90 wt %, more preferably approximatelyfrom 35-80 wt %, and most preferably approximately from 45-67 wt %.

Monomers, crosslinkers, and initiators can be incorporated in one ormore of the phases. The surfactant may comprise one or more amphiphilicblock copolymers with different functional groups. The block ratio maydiffer in the block copolymer that leads to different pore size in thefinal membrane. The aqueous to organic phase ratio and block copolymercan each be varied to form different mesostructures. Templates can beformed at different temperatures. The block copolymer is preferablycommercially available and self-assembles in the presence of twoselective solvents (e.g. water and a monomeric phase); at the end, onlythe water is preferably removed. Therefore, no complicated etching ordegradation step is needed.

Alkyl acrylates or alkyl methacrylates monomers, such asethylhexylacrylate, butyl acrylate, hexyl acrylate, octyl acrylate, nonylacrylate, decyl acrylate, isodecyl acrylate, tetradecyl acrylate, benzylacrylate, nonyl phenyl acrylate, hexyl methacrylate, 2-ethylhexylmethacrylate, octyl methacrylate, nonyl methacrylate, decylmethacrylate, isodecyl methacrylate, dodecyl methacrylate, tetradecylmethacrylate, and octadecyl methacrylate may be used as the monomer formaking regular membranes.

Functional monomers with quaternary ammonium groups may be used to makeantimicrobial membranes. Nonlimiting examples of such monomers include[2-(acryloyloxy) ethyl] trimethylammonium chloride, (3-acrylamidopropyl)trimethylammonium chloride, [2-(methacryloyloxy)ethyl]trimethylammoniumchloride, [3-(methacryloylamino)propyl]trimethylammonium chloride, and(vinylbenzyl)trimethylammonium chloride. When using a functionalmonomer, the final membrane preferably has quaternary ammonium groups onboth its surface and in its bulk, thereby making the final membranesantimicrobial.

The family of dimethacrylate, diacrylate, and divinyl crosslinkers,including but not limited to ethylene glycol dimethacrylate, ethyleneglycol diacrylate, poly(ethylene glycol) diacrylate (with differentethylene glycol repeating units), di(ethylene glycol) diacrylate,di(ethylene glycol) dimethacrylate, tetra(ethylene glycol) diacrylate,1,3-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,4-butanedioldiacrylate, tri(ethyleneglycol) diacrylate, triethylene glycoldimethacrylate, poly(ethylene glycol) dimethacrylate (with differentethylene glycol repeating units), tetraethylene glycol dimethacrylate,1,3-butanediol dimethacrylate, 1,6-Hexanediol dimethacrylate,1,4-butanediol dimethacrylate, and divinylbenzene may be used ascrosslinker. This crosslinking comonomer, or crosslinker, is preferablyadded to confer strength and resilience to the resulting membranes.

Thermal initiators such as azobisisobutyronitrile, benzoyl peroxide,tert-butyl peroxide, lauroyl peroxide, and cyclohexanone peroxide mayoptionally be used. Photo-initiators such as 1-hydroxycyclohexyl phenylketone, benzophenone, benzoin methyl ether, and benzoin isobutyl ethercan optionally be used.

The block copolymers preferably comprise amphiphilic block copolymerssuch as poly(ethylene oxide)-b-poly(propylene oxide) block copolymersfamily, poly(ethylene glycol)-block-polylactide methyl ether,poly(styrene)-block-poly(ethylene glycol), andpoly(butadiene)-b-poly(ethylene oxide). The block copolymers' molecularweights may vary. Small molecule surfactants, such asdodecyltrimethylammonium bromide (DTAB), myristyltrimethylammoniumbromide, trimethyloctadecylammonium bromide, trimethyloctylammoniumbromide, cetyltrimethylammonium bromide (CTAB), anddidodecyldimethylammonium bromide, are preferably not used.

Templates with different structures, such as lamellar, hexagonal,gyroid, or bicontinuous cubic, known as mesophases, are producedpreferably by mixing block copolymer, deionized water, and the monomericphase. At lab scale, all three phases are preferably mixed at once in aglass vial and centrifugation (2000 rpm) in alternative directions (1minute for each direction) is preferably performed about 4 to 10 timesuntil a transparent and homogeneous gel is obtained. At the industrialscale, a mechanical mixer with a high shear rate can be used to mixcomponents. The mixture, preferably comprising formed mesophases, ispreferably coated on a preferably nonwoven support (as shown in FIG. 1), preferably having a thickness of about 10 to 200 micrometers.Calendering, blade coating, casting, or any other coating method may beused. An ultraviolet (UV) light may optionally pre-polymerize themesophases for 10 minutes; then the mesophases are subsequently heatedat a temperature of about 60 to 75° C. is preferably provided tofinalize the curing, preferably for about 2 to 4 hours.

Using a functional monomer, the final membrane of the present inventionpreferably comprises quaternary ammonium groups on both its surface andin its bulk. Quaternary ammonium groups can efficiently kill microbes;therefore, the final membranes are antimicrobial.

Example 1

Several membranes were manufactured using [2-(acryloyloxy) ethyl]trimethylammonium chloride (AEAC), the structure of which is shown inFIG. 2 , as the main monomer. This monomer comprises quaternary ammoniumgroups that make it antibacterial. E. coli count tests using 3M'sPetriFilm E. coli counter were performed in order to test theantibacterial properties of the samples. To prepare the samples,as-synthesized membranes were kept in contact with a water samplecontaining E. coli for 1 hour. Then, a drop of the water sample wastransferred to the middle of the PetriFilms. The control sample was thewater sample without any contact with membranes. Different dilutions ofwater were used as shown in FIG. 3 . All PetriFilm samples were kept inan incubator at 35° C. for 48 hours.

In order to inoculate the PetriFilms, several steps were followed. (i) APetrifilm plate was placed on a level surface. (ii) The top film waslifted. (iii) A sample was measured using a micropipette, and with thepipette perpendicular to PetriFilm plate, 1 ml or 0.1 ml of the samplewas placed onto the center of the bottom film. (iv) The top film wascarefully rolled down to avoid trapping air bubbles. (v) With the flatside down, a spreader was placed on the top film over the inoculum. (vi)Pressure was applied gently on the spreader to distribute the sampleevenly over a circular area. (vii) The spreader was lifted and the gelwas given at least one min to solidify. (viii) The plate was incubatedwith the clear side up.

After 48 hours of incubation, the number of bacterial colonies in eachPetrifilm was counted using a standard colony counter. FIGS. 4A-4D andTable 1 show the number of colonies in each sample. These results showthat the as-synthesized membranes of the present invention kill almostall of the E. coli bacteria in the water sample.

TABLE 1 Sample # of colonies 1:1 (control) 560 1:10 (control) 310 1:1000(control) 100 1:10,000 (control) 30 1:100,000 (control) 2 1:1 (membrane)2 1:10 (membrane) 0 1:100 (membrane) 0 1:1000 (membrane) 0 1:10,000(membrane) 0 1:100,000 (membrane) 0

Example 2

Self-assembled mesostructures of a commercially available Pluronic®block copolymer, Pluronic® L64, in water and an oil phase consisting ofmonomers were employed as templates for the preparation of UF membraneswithout the need for organic solvents. Hexagonal and lamellar mesophaseswere prepared by changing the concentration of Pluronic® L64 and thewater/monomer ratio. Polymerization of the monomer phase via athermal/photo-initiation system followed by extraction of the aqueousphase generally retained the template structure, generating pores in theresultant membrane, as shown in FIG. 5 . Membrane performance was foundto be superior to that of a commercial NIPS UF membrane (GE, PT Series,PT8040F30) with similar pore size in terms of permeability, fluxdecline, and rejection of bovine serum albumin.

Pluronic L64, defined as poly[(ethylene oxide)13-block-(propyleneoxide)30-block-(ethylene oxide)13] (Mw=2900 g/mol) was provided by BASF.Deionized (DI) water (0.055 μs/cm, EMD Millipore Direct-Q3) was used asthe aqueous phase. Butyl acrylate (≥9%, Sigma-Aldrich) and ethyleneglycol dimethacrylate (purified, Electron Microscopy Sciences) were usedas the monomer and crosslinker, respectively. Azobisisobutyronitrile(AIBN, 98%, Sigma-Aldrich) and 1-hydroxycyclohexyl phenyl ketone (HCPK,99%, Sigma-Aldrich) were used as thermal and UV initiators,respectively. Bovine serum albumin (BSA), from Sigma-Aldrich, was usedas a solute for rejection tests. Poly(ethylene glycol) (PEG300,Sigma-Aldrich) was used as a model foulant. For molecular weight cut-off(MWCO) experiments, polyethylene glycol (PEG) with different molecularweights (200, 400, 600, 1000, 1500, 4000, and 6000 g/mol) were purchasedfrom Alfa Aesar. All chemicals were used as received without furtherpurification.

Pluronic L64, water, and an oil phase, which consisted of monomer,crosslinker, and initiator, were mixed in a glass vial viacentrifugation. In this process, samples were repeatedly centrifuged(2000 rpm) and rotated until a transparent mesophase was obtained. Itshould be noted that this alternating centrifugation method is aneffective mixing procedure and does not lead to phase stratification formixtures wherein the components have similar densities, as is the casefor the present system. Four different compositions or formulations forthe synthesized membranes, shown in Table 2, were chosen based on thelamellar and hexagonal regions of a similar phase diagram shown in P.Alexandridis, U. Olsson, B. Lindman, Self-Assembly of Amphiphilic BlockCopolymers: The (EO)13(PO)30(EO)13-Water-p-Xylene System,Macromolecules. 28 (1995) 7700-7710.

TABLE 2 Pluronic/water/oil ^(a) Membrane composition (wt. %) Mesophasestructure 1 60/30/10 Lamellar II 50/35/15 Lamellar III 60/15/25Hexagonal IV 55/15/30 Hexagonal ^(a) The oil phase consisted of monomer,crosslinker, HCPK, and AIBN, where the crosslinker, HCPK, and AIBNconcentrations were 33, 5, and 5 wt. % with respect to the monomer,respectively.

A cross-polarized Olympus microscope (model BX60) was used tocharacterize the liquid crystalline structure of mesophases before andafter the polymerization. A small amount of each mesophase (beforepolymerization) was placed on a glass slide and covered with a glasscover slip. Cross-polarized images of samples were recorded using amicroscope-mounted digital camera.

For mesophase characterization, small angle X-ray scattering (SAXS)samples were loaded into quartz capillaries with a nominal diameter of1.5 mm (Charles Supper Company, Natick, Mass.) by centrifugation.Capillary tubes were then sealed using critoseal and epoxy glue (JBWeld). SAXS measurements were performed utilizing a Bruker NanostarSystem with a monochromated Cu Kα radiation source. The beam center andsample to detector distance were determined using silver behenate.One-dimensional (1D) scattering profiles were produced through azimuthalintegration of the two-dimensional (2D) scattering patterns.

To cancel effects arising from the membrane support, membranes wereprepared using a polyethylene nonwoven fiber support recovered from thecommercial UF membrane (GE, PT Series, PT8040F30; which is used forcomparison) via a Soxhlet extraction with chloroform. Following removalof the polyethersulfone layer, recovered supports were dried under highvacuum for at least 24 hours and found to have an average thickness of190 μm. A small amount of the unpolymerized mesophase (˜2 mL) wassubsequently placed onto the support such that the mesophase comprisedroughly 60 wt. % of the final membrane. The gel mixture on the supportwas then sandwiched between Mylar sheets and smooth stainless-steelplates. The entire assembly was subsequently pressed using a hot presspre-heated to 40° C. with a force of 10⁵ N for five minutes, allowingthe monomer mixture to completely infuse the support film. The film wasthen placed in a UV chamber (Spectroline Corporation, Select XLE-Series)for 2 hours where it underwent UV polymerization, after which it wastransferred to a drying oven at 70° C. for 3 hours to ensure thepolymerization was complete. It should be noted that the supportmaterial could easily be replaced with more renewable alternatives andis not believed to have a significant effect on the membranenanostructure as evidenced by SAXS results from polymerized mesophaseswith and without the support, shown in FIGS. 6A and 6B.

After polymerization of the mesophases, SAXS measurements were performedas described above to investigate possible changes in the mesophasestructure during polymerization. In addition, mesophases werepolymerized while on a glass slide and imaged using cross-polarizedlight microscopy as described above to assess any changes to themesophase structure during polymerization.

Membrane permeability was determined using a high pressure stirred cell(Sterlitech Corporation) in a dead-end filtration mode with stirring(750 rpm). Darcy's law was used to calculate the permeability asfollows:

$\frac{\kappa}{l} = \frac{Q\mu}{A\Delta P}$

where Q, μ, A, ΔP, l, and K are the flow rate, viscosity, membrane area,pressure difference along the membrane, membrane thickness, and Darcy'sconstant (intrinsic permeability), respectively. The ratio of κ/l wasconsidered as an indication of operational permeability in this work dueto modest thickness variation between synthesized membranes, which maybias direct comparisons between different membranes. DI water wasfiltered through the membranes under 1.5 bar applied N2 pressure.Membrane effective area was constant in all samples, 14.6 cm². As notedpreviously, a commercial UF membrane (GE, PT Series, PT8040F30) with amolecular weight cut-off (MWCO) of 5 kDa (pore size of ˜2.9 nm) was usedfor comparison in this study.

To evaluate membrane separation capability, a BSA solution was used asthe feed. 1 mg/mL BSA solution in water was prepared and passed throughmembranes in a dead-end filtration mode with stirring (750 rpm).Concentration of solute in the feed and permeate were measured using aUV-Vis spectrophotometer (UV-1800, Shimadzu). Solute rejection (r) wascalculated based on the following equation:

$r = {( {1 - \frac{C_{p}}{C_{f}}} ) \times 100\%}$

where C_(p) and C_(f) are the concentrations of permeate and feed,respectively.

A Sterlitech cell was used to determine the fouling resistance ofsynthesized membranes. Solutions of 1 mg/mL BSA and 1 mg/mLpoly(ethylene glycol) with a molecular weight of 300 g/mol (PEG300) wereused as the feed. Fouling tests were performed over a period of 12 hoursin the dead-end flow configuration. The 750 rpm stirring was used duringthe fouling tests. The permeate volume was collected in 10-30 minintervals. Permeate flux was calculated and plotted against collectiontime to assess the flux decline over time due to fouling.

To determine the MWCO of membranes, 1 mg/mL aqueous solutions of PEGwith different molecular weights (200-6000 g/mol) were passed throughthem. A total organic carbon (TOC) analyzer (Shimadzu, TOC-L series) wasused to determine the PEG concentration in the permeates, and therejection values were calculated from the above equation. Each TOCmeasurement was performed 5 times and average values were reported. MWCOwas defined as the molecular weight of the PEG molecule that gives a 90%rejection.

Shown in FIG. 7 are cross-polarized micrographs obtained for mesophasesbefore and after polymerization. The streaky oil textures highlighted inFIG. 7 for both samples I and II before polymerization are indicative ofa lamellar structure, while the fan textures of samples III and IV arecharacteristic of hexagonal liquid crystals. Following polymerization,the absence of extinction (a dark image) indicates that the structureremains anisotropic. As such, these results suggest that all mesophasesretain a birefringent structure after curing. SAXS measurements furthervalidate this finding. SAXS data obtained for lamellar and hexagonalmesophases before and after polymerization are shown in FIGS. 8A-8D.FIGS. 8A and 8C correspond to compositions I and III, respectively,before polymerization. FIGS. 8B and 8D show scattering profiles forcompositions I and III, respectively, after polymerization. Scatteringprofiles for compositions II and IV compositions can be found in FIGS.9A-9D. Lamellar structures have 1:2:3:4 . . . relative peak positions(q/q*), while hexagonal mesostructures have relative peak positions of1:√3:2:√7:3 . . . , where q* is the principle peak. Therefore, based onthe scattering profiles presented in FIGS. 8A-8D and 9A-9D, samples Iand II have a lamellar structure while samples III and IV appear to behexagonal mesophases. It should be noted, however, that the anisotropyevident in the 2D scattering pattern displayed in the inset of FIG. 8Ccould indicate the presence of a hexagonal modulated (HM) or hexagonalperforated lamellae (HPL) phase, often found in close proximity to apurely hexagonal phase. This is further confirmed by the presence ofpre- and post-primary shoulders in the 1D scattering profile, indexed inFIG. 10 , which provide further evidence that the pre-polymerizedmesophase observed for sample III is an HM/HPL structure. Despite this,as can be seen in FIG. 8D, upon polymerization sample III adopts apurely hexagonal morphology.

From the scattering profiles, the pore size of the mesophase-templatedpolymers can be determined using Bragg's law, 2d sin θ=nλ, where λ isthe X-ray wavelength, θ is the scattering angle, n is the order ofreflection (taken as 1 for the principal scattering vector, q*), and dis the lattice parameter. The magnitude of the scattering vector, q, is

$q = \frac{4\pi\sin\theta}{\lambda}$

For a lamellar structure, the lattice parameter, d, also known as thelamellar periodicity, can thus be defined as follows:

$d = \frac{2\pi}{q^{*}}$

Further, for hexagonal mesophases, the lattice parameter, a, which isequal to the distance between the centers of adjacent cylinders, can becalculated as

$a = \frac{4\pi}{\sqrt{3}q^{*}}$

Calculated lattice parameters for lamellar and hexagonal mesophasesbefore polymerization (BP) and after polymerization (AP) are presentedin Table 3.

TABLE 3 Mesophase d or a (nm) δ or R (nm) Pore size Sample structure BPAP ϕ BP AP φ_(W) φ_(M) (nm) I Lamellar 8.5 9.2 0.48 4.1 4.4 0.30 0.112.8 II Lamellar 7.4 7.8 0.46 3.4 3.6 0.36 0.16 2.8 III Hexagonal 10.410.7 0.68 4.2 4.3 0.13 0.38 4.0 IV Hexagonal 10.2 10.4 0.64 3.8 3.9 0.150.31 4.2

It can be seen that the lattice parameter increases upon polymerization.We can define the apolar domain volume fraction, ϕ, as the volumefraction of the monomer phase and the PPO block, and the polar domainvolume fraction, 1−ϕ, as the volume fraction of water and thepoly(ethylene oxide)(PEO) block. Knowing the lattice parameter andvolume fractions, we can calculate the thickness of the apolar domain inthe lamellar mesophases (8) and in the hexagonal mesophases (R),illustrated schematically in FIG. 5 , as follows:

δ=dϕ

$R = {a\lbrack {1 - ( {\frac{\sqrt{3}}{\pi}( {1 - \phi} )} )^{1/2}} \rbrack}$

To calculate the volume fraction of each phase, we assume that thewater, PEO, poly(propylene oxide) (PPO), and monomer phases arecompletely segregated and that each component is characterized by itsbulk density. We note that this is not rigorously accurate, as the PEOand PPO will partition into the water and oil phases, respectively.However, these assumptions greatly simplify our calculations withoutlosing a great deal of information. For Pluronic L64, the PPO blockconstitutes 60% of the block copolymer's weight. Assuming PPO and PEO tobe at bulk density (PPO-1.005 g/cm³ and PEO-1.11 g/cm³), we can concludethat approximately 62% of polymer volume is the PPO block while PEOmakes up the other 38% of Pluronic L64 volume. For the lamellar andhexagonal samples shown in FIGS. 8A-8D, the volume fraction of PluronicL64 is thus 0.59 and 0.49 for samples I and III, respectively. Volumefractions of water, φ_(w), and monomer phases, φ_(M), in lamellar andhexagonal samples are reported in Table 3. The thickness or radius ofthe apolar domain was also calculated for each sample. This model isdepicted schematically in FIG. 5 .

To calculate the membrane pore size, we assume that the pores constitutethe space left vacant by the removal of the water. The volume fractionof water, φ_(w), in the polar domain of lamellar and hexagonalmesophases are reported in Table 3. The height of the rectangular poresin the lamellar samples, H_(p,lam), and the diameter of cylindricalpores in the hexagonal samples, D_(p,hex), can thus be calculated asfollows:

${H_{P,{lam}} = {\varphi_{w}d}}{D_{P,{hex}} = {a( {\frac{\sqrt{3}}{2\pi}\varphi_{w}} )}^{1/2}}$

As shown in Table 3, all samples were found to have a pore size of lessthan 5 nm. Table 3 also shows that polymerization results in a modestincrease in both the lattice parameter and apolar domain size. Bothdecrease and increase in the domain spacing upon polymerization havebeen reported in the literature. The former has been explained in termof a density change within the polymerized region, whereas the latter isattributed to changes in the original LLC structure. The observedincrease in the domain size may be attributable to a competition betweenthermodynamics and kinetics. On one hand, as the polymerizationproceeds, the molecular weight of the oil phase and consequently, χN, ameasure of the enthalpic penalty of mixing, approaches infinity. Thisdramatic increase in the enthalpic penalty can drive the system towardsphase separation, leading to an increase in the domain size. Changes inthe surface energy of the polymerizing phase can also lead to a phasetransition or inversion in the self-assembled structure. On the otherhand, in our system, the polymerization results in a density increase(shrinkage with Δρ˜10%). Since crosslinking arrests molecularrearrangement, trapping the structure in a non-equilibrium morphologycan be achieved if the self-organization kinetics of Pluronic are slowerthan the reaction kinetics. As such, the reaction kinetics andcrosslinker content are of critical importance for limitingpolymerization induced phase inversion/transition. Additionally, thisexplains the observed transition from an HM/HPL morphology to ahexagonal one. As shown in Table 3, expansion of the lattice parameterand apolar domain size is less significant when the monomer volumefraction increases. We attribute this to a higher shrinkage of the oilphase upon polymerization since the volume reduction is directlyproportional to the overall concentration of the oil phase.

In contrast to the above discussion, a slight decrease or increase inthe domain size with increasing temperature has been observed for theunpolymerized mesophases. It should be noted that this behavior isrepresentative of the equilibrium self-assembly and not the morphologyinduced by the polymerization described above. It is evident from FIG.11 , which displays 1D SAXS profiles as a function of temperature forsample I, that the mesostructure appears stable during the temperaturesweep and does not exhibit thermally induced phase separation (i.e. themeasurement duration is shorter than the time required forpolymerization). The observed decrease in the principal scatteringvector (increase in domain size) with increasing temperature can beattributed to a change in the surface energy of the polymerizing phase.Additionally, this result provides evidence that the elevatedtemperature for the thermal initiation process should not have asignificant effect on the nanoscale structure of the membrane. Thelimited morphological changes observed by SAXS and polarized lightmicroscopy suggest that the wide stability window of the studiedmesophases and rapid arrestment of morphological changes throughcrosslinking make this process fairly robust. This is a significantfinding as elimination of polymerization induced phase separationgenerally requires the use of polymerizable surfactants. The method ofthe present invention is both scalable and flexible, utilizing acommercially available surfactant. Additionally, monomer chemistry andsurfactants are easily adaptable, enabling production of membranestailored in terms of surface chemistry and pore size.

TABLE 4 Commercial Membrane Membrane Membrane Membrane Parameter Unitmembrane ^(a) I II III IV Q L/hr 0.12 ± 0.01 0.25 ± 0.01 0.24 ± 0.020.21 ± 0.03 0.22 ± 0.01 κ/l 10⁻¹⁰ L/m² 1.47 ± 0.01 3.05 ± 0.01 2.97 ±0.01 2.60 ± 0.01 2.69 ± 0.01 BSA % <68% >95% >95% >95% >95% rejectionTortuous — N/A (6.3 ± (7.70 ± (7.6 ± (1.1 ± (effective) 0.30) × 10⁻⁷0.7) × 10⁻⁷ 1.2) × 10⁻⁸ 0.1) × 10⁻⁷ path length ^(a) The commercialmembrane was a GE, PT8040F30.

The flow rate, normalized permeability, BSA rejection, and tortuosity ofas-synthesized lamellar and hexagonal membranes compared to a commercialmembrane are tabulated in Table 4. Both the lamellar and hexagonalmembranes were found to have substantially higher permeabilities thanthe commercial membrane, with the lamellar membranes displaying thehighest permeabilities despite a smaller characteristic pore size. Thegeometry of the nanostructure thus helps to determine the flowproperties of the membranes. While lamellar mesophases impose 1D flowconfinement (slit shaped channel), the flow in hexagonal structures isconfined in 2D (cylindrical channels). Because 1D confinement providesmore degrees of freedom for the fluid flow than 2D confinement, thehigher permeability of lamellar membranes is expected.

Tortuosity, T, is defined as the ratio of effective path for water flow,l_(e), to the thickness of the membrane, 1, i.e. T=l_(e)/l. For randomlyoriented lamellar and hexagonal channels, the tortuosity has beencalculated as 1.5 and 3, respectively, whereas it is equal to 1 forperfectly aligned channels.

We can determine the tortuosity of membranes and shed light on the fluxdifferences observed between lamellar and hexagonal mesophases. Thepermeation of water through lamellar and hexagonal channels isconsidered as water flow through a slit and a tube, respectively. Theircorresponding volumetric flow rates are defined as follows:

${Q_{e,{lam}} = \frac{\Delta{PH}_{P,{lam}}^{3}W}{12\mu l_{e}}}{Q_{e,{hex}} = \frac{\pi\Delta{PD}_{P,{hex}}^{4}}{128\mu l_{e}}}$

where Q_(e,lam) and Q_(e,hex) are volumetric flow rates for one channelin lamellar and hexagonal membranes with pore sizes of H_(p,lam) andD_(P,hex), respectively. ΔP, μ, and W are the pressure difference alongthe membrane thickness, water viscosity, and lamellar grain size (˜0.1μm, this estimate will be discussed later), respectively. Q_(e,lam) andQ_(e,hex) were calculated by dividing the total measured volumetric flowrates from Table 4 by the number of channels per membrane surface, N.For the lamellar and hexagonal structures, the number of channels permembrane surface are calculated as follows:

${N_{lam} = \frac{A_{m}}{Wd}}{N_{hex} = \frac{2A_{m}}{\sqrt{3}a^{2}}}$

where A_(m) is the membrane area. Note this calculation assumes allchannels are oriented as depicted in FIG. 5 , whereas, in reality, somechannels may be laying perpendicular to the surface. Effective pathvalues for each membrane has been calculated in Table 4. At the samemembrane thickness, the tortuosity value is proportional to theeffective path. It can be seen that, despite exhibiting higher flowrates than their hexagonal counterparts (III & IV), the lamellarmembranes (I & II) displayed a more tortuous path. This observation canbe attributed to the difference in flow geometry.

Grain boundaries in ordered phases can lead to dead-end pores andrestrict flow, affecting the tortuosity, as shown schematically in FIG.12 . FIG. 5 shows only a single unit of each mesophase type. In reality,these units stack and may orient in different directions to form theultimate structure. Using the Scherrer relation,

${{{grain}{size}} = \frac{5.56}{\Delta q}},$

we estimate me grain size across samples to be ˜0.1 μm based on thefull-width at half-maximum (FWHM) of the principal scattering peak (Δq).It should be noted that this calculation neglects the effects ofparacrystallinity, temperature, and strain on the FWHM. Therefore, thegrain size is likely underestimated in our calculation. Nonetheless,this calculation allows us to estimate that the membranes have on theorder of 100 grains across the thickness of the membrane. In polymerblends containing ion-conducting domains, it has been shown that two outof three grain boundary orientations in lamellar structures wereeffective for ion diffusion, while just one out of three cylinderorientations in hexagonal structures was efficient. As such, the lowerfluxes observed for the hexagonal mesophases over their lamellarcounterparts can be rationalized by the higher probability of dead-endsat grain boundaries in hexagonal mesophases. Further, these resultssuggest that increasing the grain size relative to the membranethickness, which can be accomplished via shear, thermal annealing, ormere reduction of the membrane thickness, can reduce tortuosity andtherefore improve the permeability of the present membranes evenfurther.

To evaluate membrane separation performance, dead-end stirred cellfiltration was performed with a 1 mg/mL BSA feed solution. UV-Visresults for the feed and permeate of commercial, lamellar (I), andhexagonal (III) membranes with a BSA feed solution are shown in FIG.13A. A calibration curve for absorbance as a function of BSAconcentration, shown in FIG. 13B, was determined and used to calculatethe concentration of BSA in each stream. The minimum detectableconcentration of BSA through this method was 0.05 mg/mL. It can be seenthat neither I or III permeates displayed a noticeable peak at 280 nm.As a result, it can be concluded that BSA concentration in the permeatesof these two membranes is less than 0.05 mg/mL. Therefore, BSA rejectionfor both lamellar and hexagonal membranes is greater than 95%. Incontrast, the commercial NIPS membrane only displayed 68% BSA rejection.

Additionally, the fouling resistance of lamellar (I) and hexagonal (III)membranes was measured and compared with the commercial membrane.Macromolecules and proteins are two of the primary foulants encounteredin water filtration. As such, 1 mg/mL BSA and 1 mg/mL PEG300 solutionsin DI water were used as foulants. Flux decline curves are shown inFIGS. 14A and 14B. Our results show that the flux declines slightly(only 6%) over 12 h for lamellar and hexagonal membranes, while there isa significant flux decline (89%) observed for the commercial membrane.These results indicate that, despite their small pore size (<5 nm), thepresent membranes display a superior fouling resistance (even in adead-end configuration) when compared to a conventional NIPS UFmembrane. This improvement can be attributed to the consistent porestructure throughout the present membranes, as well as thehydrophilicity of the Pluronic's PEG block decorating them, which is instark contrast to the anisotropic structure of NIPS membranes. Thisanisotropy as well as the low surface porosity of NIPS membranes resultsin reduced fouling resistance. In addition, the fouling resistance ofthe present membranes is further enhanced by the hydrophilicity of thepore surfaces, due to retained PEG chains. Additionally, the similaritybetween the flux decay profiles for both BSA and PEG300 is strikingconsidering BSA was found to be completely rejected, while the PEG300was found to pass through the membrane (as will be discussed). This canbe attributed in part to the high stir rate used during theseexperiments, which prevented a substantial over-layer of BSA fromforming. However, the stark contrast between the present membranes andthe commercial membrane suggest, as noted above, that the high foulingresistance of the present membranes also contributed.

FIGS. 15A-15B shows the MWCO graphs for lamellar and hexagonalmembranes. In both cases, the rejection values for M=1500 g/mol aregreater than 89.5%. Thus, the MWCO is ˜1500 g/mol. The PEG Stokesradius, α_(stokes) (nm), can be calculated as follows [39]:

α_(stokes)=16.73×10⁻¹⁰ M ^(0.557)

where M is the molecular weight of PEG. For M=1500 g/mol, the PEGdiameter is 2.0 nm, which is close to the pore size of the membranescalculated in Table 3. This suggests that, as was assumed in thecalculations above, the block polymer is retained in the pores. Themodest difference between the calculated pore size and the separablesolute size can be attributed in part to the assumption that the phaseswere completely segregated.

FIG. 16 is a cross-sectional SEM image for membrane I showing an averagethickness of 10±2 μm for the membrane.

In conclusion, self-assembled mesostructures of a surfactant in thepresence of water and oil were used as templates for the production ofUF membranes without the need for organic solvent. Cross-polarized lightmicroscopy and SAXS confirmed the retention of hexagonal and lamellarmesophases after polymerization for most samples, with only a modesttransition from a HM/HPL to a hexagonal morphology observed for samplewith Pluronic/water/oil 60/15/25 composition. As-synthesized membraneswere found to have excellent permeability with operationalpermeabilities double that of a commercial NIPS UF membrane.Additionally, the membranes exhibited MWCO of 1500 g/mol withexceptional rejection performance, >95% of BSA in a 1 mg/mL feed.Notably, the flux decline observed for both lamellar and hexagonalmembranes with 1 mg/mL BSA and PEG300 solutions over 12 hr was minimal,indicating substantial fouling resistance. Consequently, it can beconcluded that membranes produced via the present approach significantlyoutperform the commercial NIPS UF membrane used in this study. As such,these results confirm that mesophase-templated membranes could providean eco-friendly and more effective alternative to conventional NIPS UFmembranes.

Note that in the specification and claims, “about” or “approximately”means within twenty percent (20%) of the numerical amount cited. As usedherein, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a functional group” refers to one or more functionalgroups, and reference to “the method” includes reference to equivalentsteps and methods that would be understood and appreciated by thoseskilled in the art, and so forth.

Although the invention has been described in detail with particularreference to the disclosed embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverall such modifications and equivalents. The entire disclosures of allpatents and publications cited above are hereby incorporated byreference.

What is claimed is:
 1. A method of manufacturing a membrane, the methodcomprising: mixing together a surfactant, an aqueous phase, and asolution comprising one or more monomers, wherein the one or moremonomers are each different than the surfactant; forming a mesophase;coating the mesophase on a porous support; and polymerizing at leastsome of the one or more monomers to form a porous membrane on the poroussupport.
 2. The method of claim 1 wherein the porous support isnonwoven.
 3. The method of claim 1 performed without the use of anorganic solvent.
 4. The method of claim 1 further comprising removingthe aqueous phase after the polymerizing step.
 5. The method of claim 1wherein the solution comprising one or more monomers further comprises acrosslinker and an initiator.
 6. The method of claim 5 wherein at leastsome of the one or more monomers are functionalized with anantibacterial group.
 7. The method of claim 6 wherein the antibacterialgroup comprises a quaternary ammonium group.
 8. The method of claim 5wherein the initiator is a thermal initiator or a photo-initiator. 9.The method of claim 1 wherein the surfactant comprises a blockcopolymer.
 10. The method of claim 9 wherein the block copolymer is ablock copolymer of poly(ethylene oxide) and poly(propylene oxide). 11.The method of claim 9 wherein the block copolymer is amphiphilic,comprising hydrophilic blocks and hydrophobic blocks.
 12. The method ofclaim 11 wherein the hydrophilic blocks form the surfaces of pores inthe membrane.
 13. The method of claim 1 wherein the surfactant does notcomprise a small molecule surfactant.
 14. The method of claim 1 whereinthe aqueous phase is deionized water.
 15. The method of claim 1 whereinthe mixing step is performed via centrifugation.
 16. The method of claim1 wherein the polymerizing step comprises polymerizing the mesophase.17. The method of claim 1 wherein the polymerizing step comprisesexposing the mesophase to ultraviolet radiation.
 18. The method of claim1 wherein the polymerizing step comprises heating the mesophase to atemperature below 100° C. for less than 5 hours.
 19. The method of claim1 wherein the membrane is antibacterial.
 20. The method of claim 1comprising choosing relative amounts of the surfactant, the aqueousphase, and the one or more monomers in order to produce a desiredstructure of the mesophase.
 21. The method of claim 20 wherein thestructure is hexagonal or lamellar.
 22. The method of claim 1 furthercomprising hot pressing the mesophase and the support to infuse themesophase into the support prior to the polymerization step.
 23. Themethod of claim 1 wherein a pore size of the membrane is less thanapproximately 5 nm.
 24. The method of claim 1 wherein a pore size of themembrane is between 2 nm and 50 nm.
 25. The method of claim 1 wherein anaverage grain size of the membrane is approximately 100 nm.
 26. Themethod of claim 1 wherein the membrane comprises monodisperse pores. 27.The method of claim 26 wherein surfaces of the pores are hydrophilic.28. The method of claim 1 wherein the membrane is isoporous.
 29. Themethod of claim 1 wherein the mixing step comprises alternatingcentrifuging and rotating.